Low current drain amplifier incorporating feedback means for establishing sensitivity

ABSTRACT

A low current drain board bandwidth amplifier circuit for detecting low level signals of positive or negative amplitude, and for producing an output pulse whenever the input signals exceed a predetermined threshold level. A first operational transconductance amplifier is used as an amplifier, followed by a pair of operational transconductance amplifiers employed as comparators. The reference voltage for each comparator is set through a resistive circuit driven by one or more current sources. Feedback means are employed with the amplifier OTA to reduce first stage offset, and sensitivity is established with minimal adjustment requirements.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention lies in the field of electronic amplifiers and, moreparticularly, amplifiers suitable for low signal threshold amplificationwith low current drain operation.

2. Description of the Prior Art

There are a wide variety of present day applications which requireamplification of low level signals under extremely low current drain, orlow power consumption conditions. For example, in the field ofimplantable electronic pacers for cardiac pacing, where the batteryenergy of an implanted pacer must be conserved to the maximum extentpossible, there exists an acute need for an amplifier which is capableof detecting when low level incoming signals exceed a predeterminedthreshold and amplifying same so as to produce a resultant output pulse,under circuit conditions which require a minimum current drain from thepacer battery. More particularly, in demand-type pacers where outputstimulus pulses are delivered only in the absence of natural pulses, thequiescent current drain from the battery is determined largely by thedesign of the amplifier portion which recognizes and amplifies theincoming natural beats and generates an output signal suitable forresetting of the pacer oscillator. For further background on thisproblem, reference is made to the copending application Ser. No.608,465, filed Aug. 28, 1975, and titled "Multiple Function Demand PacerWith Low Current Drain" and assigned to the same assignee.

It has long been considered virtually impossible to design an amplifiersuitable for use in an implanted pacer which would draw only on theorder of 1 microamp, but this has remained a standard which the industryhas hoped to achieve. At the same time, any successful amplifier designmust provide that the characteristics will be essentially the same inthe environment where the amplifier is to be used as they are when theyare tested. In the specific example of electronic pacers, one of thecritical requirements is that the drift in threshold level, orsensitivity, from the temperature at which tested as compared to theoperational temperature of about 37° C, be minimized to the greatestpossible extent. Also, to insure uniformity of amplifier characteristicsfor production devices, it is desired that the amplifier design providemaximum independence of the normal variation in components, so as toavoid the increased expense of utilizing extremely low toleranceelements or difficult adjustment procedures.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following applications are filed concurrently herewith, are assignedto the same assignee, and are incorporated by reference:

1. MULTIPLE-FUNCTION DEMAND PACER WITH LOW CURRENT DRAIN, Invention ofAlexis C. M. Renirie Ser. No. 608,465, filed Aug. 28, 1975.

2. LOW CURRENT DRAIN AMPLIFIER INCORPORATING MEANS FOR MINIMIZINGSENSITIVITY DRIFT, Invention of Alexis C. M. Renirie Ser. No. 608,529,filed Aug. 28, 1975.

3. LOW CURRENT DRAIN AMPLIFIER WITH SENSITIVITY ADJUSTMENT MEANS,Invention of Alexis C. M. Renirie and Godefridus J. M. WEijs Ser. No.608,587, filed Aug. 28, 1975.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an amplifier circuit havingthe capability of detecting input signals of only several millivolts(mV) and for producing an output signal of desired magnitude andwaveform corresponding to input signals above a predetermined threshold,while simultaneously blocking lower level signals, all while maintainingquiescent current drain as low as possible.

It is a further object of this invention to provide an amplifier circuitfor providing high gain amplification of input signals above apredetermined threshold level, the circuit being substantiallyinsensitive to variations of the power supply and being relatively easyto adjust for accurate threshold detection.

It is another object of this invention to provide an amplifier circuitfor providing a predetermined output drive signal upon receiving aninput signal of either positive or negative polarity which exceeds apredetermined threshold level, which amplifier operates with a currentdrain of only about 1 micro-ampere (1 uA), which is substantiallyindependent of circuit component variations and the condition of thepower supply, has minimal drift in operating characteristics as afunction of temperature, and is simple to adjust in order to obtain thedesired threshold characteristics.

In accordance with the above objectives, there is provided an amplifiercircuit having a first amplifier stage including an operationaltransconductance amplifier (OTA), a comparator stage comprising a pairof operational transconductance amplifiers connected in inverting andnon-inverting modes and being operated with a bias or reference voltageacross their differential inputs which, together with the amplifierstage, establishes the desired sensitivity. In order to preciselyestablish the desired threshold and to compensate for the offsetinherent in each operational transconductance amplifier, means areprovided including a current source in combination with a predeterminedresistive network for establishing desired bias voltages at thecomparator amplifiers. The threshold establishing, or sensitivitycircuits, are designed to maximize ease of the adjustment procedure forestablishing the sensitivity, and to minimize drift in the sensitivitycharacteristics as a function of the ambient operating temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic drawing of the basic components of theamplifier circuit of this invention.

FIG. 2 is a circuit diagram showing a first embodiment of the amplifiercircuit of this invention utilizing four OTAs and a single FET currentsource.

FIG. 3 is a circuit diagram of a second embodiment of the amplifiercircuit of this invention, which provides for adjustment of sensitivitythrough adjustment of the bias voltage on the two output OTAs.

FIG. 4 is a circuit diagram of a third embodiment of the amplifiercircuit of this invention, which utilizes one amplifier OTA and twocomparator OTAs, and having a bridge circuit adapted for adjustment ofthe overall circuit sensitivity and driven by a current source.

FIG. 5 is a modification of the three OTA embodiment of FIG. 4, withcircuitry for establishing the overall sensitivity with a decreasedadjustment requirement.

FIG. 6 is another embodiment of the amplifier circuit of this inventionutilizing a single OTA amplifier stage and inverting and non-invertingOTA comparator stages driven by the amplifier stage, with a feedbackpath in the amplifier stage to reduce the effect of first stage offset,thereby reducing the criticality of the sensitivity adjustment and thedegree of sensitivity drift.

FIG. 7 is a modification of the feedback circuit of FIG. 6, with analtered first stage feedback arrangement which provides higher gainoperation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown the basic configuration aroundwhich the various embodiments of the amplifier circuit of this inventionare constructed. As shown, there is a first operational transconductanceamplifier (OTA) 45 which operates basically as an amplifier stage. TheOTA acts as a current source at its output, providing an output currentwhich is proportional to the differential input voltage placed acrossits two input terminals. The polarity of the output current is dependentupon the polarity of the differential input voltage. The OTA has aspecial bias current terminal for receiving bias current I_(ABC), whichbias current determines the essential characteristics of the OTA, andparticularly the transconductance and the current drain. The totalcurrent drain of an OTA, including I_(ABC), is 3 × I_(ABC), such thatthe current drain of the device can be established by fixing I_(ABC). Asuitable operational transconductance amplifier for use in thisinvention is the RCA linear integrated circuit CA3080. Reference is madeto the RCA literature for a full description of the characteristics ofthis device. Reference is also made to the RCA application noteICAN-6668, dated November 1973, which notes provide material explanatoryof the circuit characteristics of the operational transconductanceamplifier. As used in claiming this invention, the output of the firststage OTA is direct connected to a second stage OTA 46 which is utilizedas a comparator device. Of the two input terminals of the second stageOTA, one receives the output of the first stage amplifier, in either aninverting or non-inverting mode, and the other input terminal receives abias voltage, the quiescent voltage difference across the two terminalsestablishing the magnitude of input swing required to cause a change inpolarity of the output signal from the OTA comparator. Thus, if theamplitude of the signal coming from the amplifier OTA is larger than afixed reference level established across the differential inputs of thecomparator OTA, and opposite in polarity, the input voltage of thecomparator OTA changes from one polarity to another, thus switching thecomparator output signal. In practice, the voltage gain of the amplifierstage is generally between 10 and 20, which gain is one of thedeterminants of the swing in the input level to the first amplifierstage which is required to switch the output of the comparator.

Still referring to FIG. 1, the output of comparator 46 is direct coupledinto the gate of the CMOS transistor 47, which as shown has its sourceconnected to ground and its drain connected to a current source 48.Because the OTA is a current source itself, and the input resistance ofa CMOS transistor is extremely high, the voltage gain of the comparatorOTA is very high. For example, for a transconductance of 2 uA/V and aninternal resistance of 75 × 10⁹ ohms, the gain is 150,000. Thisextremely high gain provides a very high switching response, such thatas soon as the input to OTA 45 exceeds a predetermined signal threshold,the output of comparator 46 switches essentially instantly from anegative to a positive current, thus driving transistor 47 conductiveand producing a voltage change at the output terminal suitable fordriving the output load connected thereto.

In practice, several problems must be dealt with in order to adapt thebasic circuit of FIG. 1 so as to provide it with the desired low currentdrain, highly stable operation that is desired. For example, all OTAdevices have some minimum amount of offset, meaning that the output doesnot switch precisely at the point when the voltage differential acrossthe input terminals is zero, but when the differential is offset by somesmall amount, generally a fraction of a mV. The offset in the amplifierstage is, of course, amplified such that it has an amplified effect atthe input to the second stage comparator OTA. Additionally, thecircuitry providing the bias, or reference voltage is, generallyspeaking, subject to variations as a function of component tolerancesand temperature changes, which must also be accounted for. Also, as isevident from FIG. 1, a single amplifier path using two OTAs is notcapable of providing a predetermined output signal in response to eithernegative or positive going input signals. These and other problems areanticipated and dealt with in the various embodiments which aredescribed in the following portion of the specification. For example, insome of the circuit embodiments, the bias voltage is supplied as theout-of-balance voltage of a resistance bridge, the input voltage drivingthe bridge to balance at which time the comparator switches. Theadvantage of this arrangement is that the overall circuit is relativelyinsensitive to variations of the supply source. However, thedisadvantage is that adjustment of the bridge has to be relativelyaccurate, in order to provide for the desired threshold, or sensitivity.

In the discussion to follow, the term sensitivity is employedsynonymously with threshold, and refers to the signal level at which theamplifier circuit of this invention provides a change at its output. Theoutput OTA operates with a class A push-pull output characteristic, suchthat its output current is positive or negative as a function of thelevel of the input relative to the bias voltage on the referenceterminal. However, the gain of the OTAs is sufficiently high that theoutput current is either an "on" or "off" current for purposes ofdriving the CMOS transistor element connected to the output of thecomparator device. Thus, when used to drive a high input load as in thisexample, the amplifier circuit provides either an on or off output, withswitching occurring at the established sensitivity, or threshold level.It is to be noted, of course, that the amplifier may also be used forlinear operations, where the waveform of the output signal issubstantially preserved instead of being converted to an on-off levelsignal, provided the input signal is in excess of the establishedthreshold. Of course, by placing the threshold at zero, the amplifiermay be used in an entirely linear operational manner.

Referring now to FIG. 2, there is shown a circuit diagram of a four OTAembodiment of the amplifier of this invention. A single current sourceis provided by connection of an FET 61 with a resistor 62 to a positivepower source. The output of the current source is connected to resistors63, 64, and 65 respectively, which are selected to provide the desireddivision of current. By adjustment of resistors 62, 63, 64 and 65, thereare effectively three current sources provided for operation of thiscircuit. The junction of resistors 63, 64 and 65 is also connectedthrough capacitor 79 to ground.

In this embodiment, there are two paths utilized, comprising amplifierOTA 53 and comparator OTA 54, and amplifier 55 and comparator 56. Theinput signal is coupled through capacitor 77 to the negative, orinverting terminal of OTA 53 and to the positive, or non-invertingterminal of 55. Current limiting resistor 63 is connected to thepositive terminal of OTA 53, and through resistors 66 and 68 to thenegative terminal of OTA 55. The negative terminal of OTA 55 is alsoconnected through resistor 71 to the current bias input terminal of OTA53 and through resistor 72 to the current bias input terminal of OTA 55.Resistor 67 is connected between capacitor 77 and the junction ofresistors 66 and 68, with junction is also connected to the twonegative, or reference terminals of comparators 54 and 56. Capacitor 78is connected between such junction and the ground. Resistor 64 isconnected to the output of OTA 55, the positive input of OTA 56, andthrough bias current resistor 74 to the bias input terminal of OTA 56.Resistor 65 is connected to the output of OTA 53, to the positive inputterminal of OTA 54, and through resistor 73 to the bias current inputterminal of OTA 54. Note that, with this arrangement, each amplifier OTAis loaded with a current source, an OTA input and a bias currentresistor in parallel, and the input circuit of each of the four OTAs isdriven with a current source.

While this configuration, as shown in FIG. 2, embodies a single FETcurrent source, it is to be understood that an equivalent circuit cancomprise three separate current sources, each comprising an FET/resistorcombination, whereby high value resistors 63, 64 and 65 can be avoided.In a plural current source configuration, the tolerance of the resistorsis not as critical as compared to the one FET arrangement shown in FIG.2, but two additional FETs are required which can contribute toincreased sensitivity drift.

Some of the more specific and detailed advantages of the circuit of FIG.2 can be understood by a further analysis of the problems associatedwith sensitivity drift. Assuming a bias voltage on the output OTAs of200 mV, and a typical gain of the first amplifier OTAs of 16, the inputdifferential voltage to each amplifier stage must be 12.5 mV in order tocause switching at the output. Examining OTA 53, the input consists ofthe quiescent voltage over resistors 66 and 67, plus the negative goingamplitude of the input signal. For negative sensitivity of 2 mV, thevoltage over resistor 66 and 67 has to be 10.5 mV. This can beestablished by adjustment of resistor 66 so that its value, for thegiven current source, contributes the required quiescent voltage at theinput. Likewise, a similar analysis pertains for OTA 55, where thepositive sensitivity can be established by adjustment of the value ofresistor 68. The adjustments of resistors 66 and 68 do not influenceeach other, due to the constant current source and the fact thatresistors 66 and 68 are small in comparison with resistors 71 and 72.The circuit has a very large adjustment range, relatively independent ofthe election of the circuit components, and can always be adjusted to asensitivity of 1 to 2 mV.

However, it is to be noted that the FET current source has a drift withtemperature. While a pacer which is implanted in a human patient ismaintained at a substantially constant temperature, it is recognizedthat the adjustment procedure on any circuit would normally be done atroom temperature, or about 22° C, following which it would be utilizedat the patient temperature of about 37°. The sensitivity drift whichaccompanies such a 15° change is defined as the 15° drift, or morebriefly simply the drift. An FET has a relatively large spread in V_(p),which is the voltage needed to cut off current flow through the device.Typically, this lies between 300 mV and 100 mV, with a temperature driftof 2.2 mV per degree centigrade. If the temperature rises 15° C, thecurrent source experiences an increase of 5% in current for a typicalV_(p) of roughly 650 mV. The temperature drift of FET 61 thus causesapproximately a 5% drift of voltage over resistors 66 and 68, whichtypically would result in a drift of about 0.53 mV. It is to be notedthat, for configurations utilizing three FETs instead of one, thedifference of V_(p) between the FETs would introduce additional drift.In any event, it is to be observed that by lowering the voltage acrossresistors 66 and 68, the amount of drift can be lowered correspondingly.This can be achieved by choosing a lower value for the bias voltage atthe comparator OTAs, which is done by adjusting resistors 64 and 65,resulting in a lower adjusted voltage over resistors 66 and 68 and aconsequent lower magnitude of drift caused by variations in FET 61. Ifthree FETs are used, bias voltage at the comparator cannot be reducedtoo much, and for each value of the spread of V_(p) for a given FETthere is an optimal value for the comparator bias voltage. The value of200 mV, as noted above, corresponds roughly with the spread which hasbeen found in practice.

If one FET is used, it is possible to reduce the bias voltage to thepoint corresponding with resistors 66 and 68 both being equal to 0, asshown in FIG. 3. In this case, the sensitivity is established byadjustment of the resistors 64 and 65. When these resistors are adjustedto provide the desired negative and positive sensitivity, all offsets ofthe OTAs are also compensated for. In this circuit, the only changecaused by temperature drift of the voltage in the input circuit is thevoltage across resistor 67A, which corresponds to resistor 67 in FIG. 2.This change in voltage is secondary and not as great as the changeacross resistors 66 and 68 of FIG. 2, and therefore the resultingsensitivity drift is much lower. For this circuit, the 15° drift is 0.1to 0.3 mV, with a current drain of about 1.2 uA. By comparison, thetypical drift of the circuit of FIG. 2 is about 0.5 mV with a ratherhigh spread. Additional offset compensation can be obtained by includingthe resistor 67B connected to the positive input terminal of OTA 53 andto the negative input of OTA 55, as shown. With this offsetcompensation, sensitivity drift is reduced to 0.1 mV or less. It is tobe noted that resistor 67A could be decreased in resistance in order tofurther reduce sensitivity drift, but this change would also reduce theinput impedance and thus adversely affect overall gain.

Referring now to FIG. 4, there is shown a circuit diagram of aconfiguration of the amplifier circuit of this invention utilizing onlythree OTA units, instead of four as in the previously discussedconfigurations. It is to be noted that in the configurations where thereare two OTA amplifiers, they both receive the same input signal,although they are received with opposite phases. For example, in FIG. 2,two amplifiers must be used because their inputs must be given adifferent DC-shift. However, the phase inversion required for thepurpose of handling both negative going and positive going signals canbe accomplished at the second, or comparator stage, while utilizing onlyone OTA at the amplifier level. This is what is done in FIG. 4, wherethere is a single amplifier OTA 53, and two comparator amplifiers 54 and56. The output of OTA 53 is connected to the positive, or non-invertinginput terminal of OTA 54, and to the negative, or inverting terminal ofOTA 56. By this technique, and by establishing the desired bias voltageson the comparator OTAs 54 and 56, the desired negative and positivesensitivities are obtained.

In the configuration of FIG. 4, the current source is again obtained byan FET 61 and resistor 62, which is connected to a common junction withresistors 63, 64 and 65. Resistor 63 is connected to the positive inputof OTA 53, resistor 64 is connected to the output of OTA 53, andresistor 65 is connected to the negative, or reference input terminal ofOTA 54 and through adjustable resistor 81 to the positive referenceterminal of OTA 56. The other circuit components are similar to andserve the same purposes as those in prior configurations. The adjustmentof the precise desired sensitivity involves setting resistance 64, whichbalances a bridge comprised of resistances 64, 65, 73 and 74. Inaddition, adjustment of resistor 81 enables adjusting of the positiveand negative sensitivity. The first stage offset may be compensated byresistor 67B, and any second stage offset is taken into account in theadjustment of resistors 64 and 81. In this configuration, the 15° drifthas been measured to be less than 0.05 mV; the current drain is 0.9 uA;and, of course, only three OTAs are utilized instead of four.

In the configurations discussed thus far, resistive bridges are employedwhich are set to be out of balance, thereby normally providing thedesired bias voltage. When the amplified input signal drives thesebridges to the balance point, the input at one of the comparator OTAdevices changes polarity, thereby producing the desired switchingsignal. Once adjustment of the bridge is achieved, the desiredsensitivity is achieved with very low sensitivity drift, and withsubstantial independence of supply voltage. By contrast, the followingcircuits do not use a bridge principle, but have a sensitivity which isdetermined by the tolerance of the resistors utilized. Resistances inthe range of about 10 megohms are available with a tolerance of about10%, which allows for designing the overall circuit characteristics witha tolerance better than most circuits presently available in the art.Furthermore, the stability of the circuit embodiments which follow issufficiently good that with adjustment an accuracy of only a few percentis easily achieved over a temperature range from 20° to 37° C.

The bridge circuits have a 15° drift which is in principle independentof the adjusted sensitivity. The non-bridge circuits have a 15° driftwhich is in principle a fixed fraction of the adjusted sensitivity. Thedrifts mentioned herein for these latter circuits correspond to asensitivity of 2 mV.

Referring to FIG. 5, there is shown a first example of a configurationof the amplifier circuit of this invention which provides a 15° driftcharacteristic in the range of 0.1 to 0.25 mV. The input signal isconnected through capacitor 77 to the negative, or inverting terminal ofOTA 53. Capacitor 77 is connected through resistance 67A to the currentsource comprised of FET 61 and resistor 62, which point is alsoconnected through resistor 67B to the positive input terminal. Theoutput of the current source is also connected to the negative terminalof comparator OTA 56, and through resistors 87 and 88 to the positiveterminal of OTA 54. Resistors 86 and 91 may also be connected as shownto the respective input terminals of the comparator OTAs, to reduce thedrift. The output of amplifier OTA 53 is connected through capacitor 84to the negative terminal of OTA 54, and to the positive terminal of OTA56. Resistor 83 is connected between the output of OTA 53 and the outputof the current source. Since the offset voltage of OTA 53 is amplifiedby the OTA itself, this voltage is blocked by capacitor 84, to avoid theinfluence of this amplification on the sensitivity. Resistor 83 isneeded only to allow capacitor 84 to discharge or charge to the desiredDC value. This resistor must be as high as possible in order to minimizeits effect on voltage gain.

The tolerance of the sensitivity, without any adjustment of the circuitresistors, is determined by the relative tolerance of resistors 87, 88,89, 90, 71 and 85. The 15° drift without resistors 86 and 91 has beenmeasured to be 0.25 mV; and the drift with resistors 86 and 91 has beenmeasured to be 0.1 mV. The current drain with resistor 71 set at 10Megohm is 0.9 uA, and with resistor 71 set at 5 Megohm, 1.2 uA. In thisarrangement, if resistors 87 and 88 are adjusted to obtain the desiredsensitivity, resistor 83 and capacitor 84 are superfluous.

Referring now to FIG. 6, there is shown a modified circuit designed toprovide the optimum accuracy of sensitivity without need for adjustment.In this circuit, the output of amplifier OTA 53 is connected throughresistor 95 to the negative input terminal, the DC component of theoutput voltage being completely fed back due to the blocking effect ofcapacitor 77. As a consequence, the offset voltage of OTA 53 is notamplified. Further, the load which appears across the output of OTA 53is extremely high, being determined by resistor 95 which is suitably 20Meg and the inputs of OTAs 54 and 56, which are each typically about 40Meg. The sensitivity drift is primarily determined by the relativetolerances of resistors 67B and 95, a relative tolerance of 10% causinga 15° drift of less than 0.05 mV. The current drain of this circuit is0.9 uA for resistor 71 at 10 Meg, and 1.15 M for resistor 71 at 6 Meg.

The circuit configuration as shown in FIG. 7 is varied from that shownin FIG. 6 in that the input is coupled directly through capacitor 77 tothe positive input terminal of OTA 53, while the feedback path from theoutput of OTA 53 is through resistor 95 to the negative terminal. Thenegative terminal is capacitively connected to ground by capacitor 101.Note that the amplifier of FIG. 7 has no AC-feedback and therefore ahigher gain than that of FIG. 6. As a result of driving the positiveinput instead of the negative input, the amplifier of FIG. 7 also has ahigher input resistance.

We claim:
 1. An amplifier circuit, for amplifying input signals and forproducing an output signal of a predetermined polarity when said inputsignal exceeds a predetermined magnitude, comprising:a. a first inputstage, having a differential input amplifier having first and secondinputs and an output with a feedback circuit from the amplifier outputto said first input of the amplifier; b. two output comparator stages,each having a negative input terminal and a positive input terminal; c.a connection circuit connecting the output of said input stage torespective opposite polarity input terminals of said two outputcomparator stages; d. current source means for providing a source ofcurrent; e. an input circuit for connecting input signals to one of saidfirst and second amplifier inputs of said first stage; and f. asensitivity connection circuit connecting said current source to saidsecond amplifier input of said first stage and to the respective otherterminals of each comparator stage.
 2. The amplifier circuit asdescribed in claim 1, wherein said sensitivity connection circuitcontains resistances of values predetermined for adjustment of theamplifier sensitivity.
 3. The amplifier circuit as described in claim 2,wherein said input stage comprises opposite polarity input terminals,and said input circuit and said feedback circuit are connected to thenegative input terminal.
 4. The amplifier circuit as described in claim2, wherein said input stage has opposite polarity input terminals, andsaid feedback circuit and said input circuit are connected to respectivedifferent ones of said input terminals.
 5. The amplifier circuit asdescribed in claim 4, wherein said feedback circuit is resistive.
 6. Theamplifier circuit as described in claim 1, wherein said input stage andsaid two output comparator stages each comprise operationaltransconductance amplifiers.
 7. The amplifier as described in claim 6,wherein said current source means comprises a single active device, andsaid sensitivity connection circuit comprises resistors connected to thebias current terminals of each of said operational transconductanceamplifiers.